Schizophrenia is a frequently chronic psychiatric illness associated with significant morbidity and loss of functioning. Two important neurotransmitter systems, dopamine and glutamate, have been implicated in the production of symptoms in schizophrenia, but the relative contributions of the two systems and of their interaction, are unknown. The purpose of this study is to learn more about the interactions between glutamate and dopamine in normal subjects. Specifically, we will examine the effects of decreased glutamate transmission on amphetamine-induced dopamine release. We propose to test the hypothesis that administration to healthy volunteers of the glutamate antagonist ketamine will produce a dysregulation of dopamine neurons similar to the dysregulation observed in patients with schizophrenia. If this hypothesis were confirmed, it would open a bridge between the dopamine and glutamate hypotheses of schizophrenia.